Parse discard binding by JLHwung · Pull Request #17163 · babel/babel

JLHwung · 2025-03-05T18:53:16Z

Q	A
Fixed Issues?	Support Discard Binding Proposal in `@babel/parser` and `@babel/generator`
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

The AST design is aligned to the current ESTree proposal: https://github.com/estree/estree/blob/master/experimental/discard-binding.md

This PR will be merged to the umbrella branch feat-discard-binding.

babel-bot · 2025-03-05T18:55:45Z

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/59219

JLHwung · 2025-03-06T16:33:51Z

...s/babel-parser/test/fixtures/experimental/discard-binding/using-variable-declarator/input.js

+}
+{
+  // using declarator in for-of
+  for(using void of []);


Although the parser currently accepts this production, it is not allowed in the draft spec. I have filed an issue tc39/proposal-discard-binding#14

Yeah this seems like a spec bug.

packages/babel-plugin-transform-object-rest-spread/src/shouldStoreRHSInTemporaryVariable.ts

packages/babel-types/src/definitions/core.ts

nicolo-ribaudo

I believe multiple delegates are still unhappy with the idea of using void. Could we maybe add a required syntax: "void" option to the parser plugin?

JLHwung · 2025-03-07T13:53:57Z

Could we maybe add a required syntax: "void" option to the parser plugin?

Sure, I will add a syntaxType option since we already have one for the record and tuple transform.

I believe multiple delegates are still unhappy with the idea of using void.

Out of curiosity, do you know if there are other type candidates? If void will not be likely adopted, then we should consider rename VoidPattern to DiscardPattern so that it is not tied to the void token.

nicolo-ribaudo · 2025-03-07T16:17:43Z

Out of curiosity, do you know if there are other type candidates? If void will not be likely adopted, then we should consider rename VoidPattern to DiscardPattern so that it is not tied to the void token.

The other possible approach is to relax the existing "duplicated declaration" errors in case where something is declared twice but not read, so that { let _ = 1; let _ = 2; } would be valid. I think the current AST is fine, as in that case we would just use an Identifier node.

JLHwung · 2025-04-16T18:18:08Z

Rebased. Now that #17217 has been merged, the last two validation harden commits are now replaced by 1a4fc60.

nicolo-ribaudo · 2025-04-25T08:47:01Z

@liuxingbaoyu Do you think you could review this in the next few days? It would be great to include it as merged in the STA report for April :)

liuxingbaoyu · 2025-04-25T20:39:19Z

Of course! I will finish it soon. :)

nicolo-ribaudo · 2025-04-25T20:41:38Z

(actually, I realized this will wait anyway because it's a new feature, but reviewing it will still unlock work on the transform!)

packages/babel-parser/src/parser/expression.ts

Co-authored-by: liuxingbaoyu <30521560+liuxingbaoyu@users.noreply.github.com>

nicolo-ribaudo · 2025-04-30T08:29:26Z

@JLHwung As this PR is ready, let's merge it to the feat-discard-binding branch so that you can open the transform PR on top of it?

* add VoidPattern definition * update types artifacts * parse void binding * print void binding * fix typing errors * update parser typings * fix: remove VoidPattern from ForXStatement.left * fix typing errors * add Babel 8 tests * require syntaxType option * add test for object-rest-spread change * feat(types): disallow void pattern in var/let/const declaration * fix: allow nested void pattern in catch param * improve UnexpectedVoidPattern error message * remove R&T integration parser tests * polish: throw different error when CoverDiscardElement is parsed * Update expression.ts Co-authored-by: liuxingbaoyu <30521560+liuxingbaoyu@users.noreply.github.com> * update discardBinding plugin option types --------- Co-authored-by: liuxingbaoyu <30521560+liuxingbaoyu@users.noreply.github.com>

JLHwung added PR: New Feature 🚀 A type of pull request used for our changelog categories Spec: Discard Binding labels Mar 5, 2025

JLHwung commented Mar 6, 2025

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JLHwung force-pushed the parse-discard-binding branch from 08d4b6c to f2dd864 Compare March 6, 2025 20:32

nicolo-ribaudo reviewed Mar 7, 2025

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nicolo-ribaudo reviewed Mar 7, 2025

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nicolo-ribaudo mentioned this pull request Mar 7, 2025

Babel 7.27.0 blog post babel/website#3062

Merged

JLHwung force-pushed the parse-discard-binding branch from 4aaac4a to a4d2855 Compare March 7, 2025 15:28

JLHwung force-pushed the parse-discard-binding branch from d2c943a to a068f39 Compare March 7, 2025 16:38

JLHwung force-pushed the parse-discard-binding branch 2 times, most recently from 21e672b to 7675a3c Compare March 24, 2025 22:32

JLHwung mentioned this pull request Mar 31, 2025

Harden variable declarator validations #17217

Merged

JLHwung added 12 commits April 16, 2025 13:59

add VoidPattern definition

43cdec3

update types artifacts

a777ae9

parse void binding

93cded5

print void binding

090be9c

fix typing errors

cd0cca9

update parser typings

5626010

fix: remove VoidPattern from ForXStatement.left

1ba8983

fix typing errors

2e204ee

add Babel 8 tests

d58ce5c

require syntaxType option

ed243b5

add test for object-rest-spread change

258012b

feat(types): disallow void pattern in var/let/const declaration

1a4fc60

JLHwung force-pushed the parse-discard-binding branch from 7675a3c to 1a4fc60 Compare April 16, 2025 18:15

JLHwung requested a review from nicolo-ribaudo April 16, 2025 18:22

polish: throw different error when CoverDiscardElement is parsed

f8d042b

nicolo-ribaudo approved these changes Apr 24, 2025

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liuxingbaoyu reviewed Apr 27, 2025

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packages/babel-parser/src/parser/expression.ts Outdated Show resolved Hide resolved

liuxingbaoyu approved these changes Apr 27, 2025

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JLHwung and others added 2 commits April 27, 2025 22:14

Update expression.ts

57176d2

Co-authored-by: liuxingbaoyu <30521560+liuxingbaoyu@users.noreply.github.com>

update discardBinding plugin option types

137501b

JLHwung force-pushed the parse-discard-binding branch from ee42611 to 137501b Compare April 28, 2025 18:22

JLHwung merged commit e8a66ca into babel:feat-discard-binding Apr 30, 2025
55 checks passed

JLHwung deleted the parse-discard-binding branch April 30, 2025 12:07

JLHwung mentioned this pull request Apr 30, 2025

Support discard binding proposal #17276

Merged

2 tasks

fisker mentioned this pull request Jul 3, 2025

Support "discard binding" prettier/prettier#17699

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github-actions bot added the outdated A closed issue/PR that is archived due to age. Recommended to make a new issue label Jul 31, 2025

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JLHwung added pkg: generator pkg: parser labels Feb 24, 2026

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